One of examples of a semiconductor device is an electrooptic display device to be used as a display device including a TFT active matrix substrate using a thin-film transistor (abbreviated as a TFT) as a switching element. Such an electrooptic display device is considered as one of flat panel displays as alternatives to a cathode ray tube (CRT) and has been applied actively to products for its features of low power consumption and thinness.
The aforementioned semiconductor device has been required to achieve cost reduction. To simplify manufacturing steps, an inversely staggered structure has mainly been employed conventionally for a TFT using amorphous silicon (Si) in a semiconductor active layer. The inversely staggered structure is called a back channel etching type or a back channel etch type.
Development has been underway actively for a TFT using an oxide semiconductor (hereinafter called an “oxide TFT” in some cases) having higher mobility than conventional amorphous silicon (see for example Japanese Patent Application Laid-Open Nos. 2005-77822 and 2007-281409 and “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Kenji Nomura et al., pp. 488-492, Vol. 432 of Nature, 2004)
Examples of a light-transmitting conductive film conventionally known include oxide conductive films such as an ITO film containing indium tin oxide (abbreviated as ITO) as a compound of indium oxide In2O3 with tin oxide SnO2 and an IZO film containing indium zinc oxide (abbreviated as IZO) as a compound of indium oxide In2O3 with zinc oxide ZnO.
Like these oxide conductive films, an oxide semiconductor film containing the aforementioned oxide semiconductor is insoluble in an alkaline developer for a photoresist and can be etched with a mild acidic solution of oxalic acid and carboxylic acid. Thus, the oxide semiconductor film can advantageously be formed into a pattern easily by wet etching process as process with a chemical.
Meanwhile, the aforementioned oxide semiconductor film easily dissolves further in a publicly-known acidic solution used for etching a metal film to become a source electrode and a drain electrode of a TFT such as a metal film containing chromium (Cr), titanium (Ti), molybdenum (Mo), tantalum (Ta), aluminum (Al), or an alloy of these metals.
Thus, for manufacture of an oxide TFT using an oxide semiconductor film in an active layer, the type of a metal film to become a source electrode and a drain electrode, that of a solution for etching of the metal film, and that of the oxide semiconductor film are selected so as to enable selective etching by which only the metal film is etched and the oxide semiconductor film remains unetched (see Japanese Patent Application Laid-Open No. 2008-72011, for example).
A technique of manufacturing an oxide TFT at low cost is disclosed for example in Japanese Patent Application Laid-Open No. 2010-123937. Japanese Patent Application Laid-Open No. 2010-123937 discloses a manufacturing method by which a process of processing a semiconductor film to become an active layer, a process of processing a metal film to become a source electrode and a drain electrode, and a process of separating an ohmic contact layer formed at a connection interface between the source electrode and an oxide semiconductor film and between the drain electrode and the oxide semiconductor film are performed in one mask step using a halftone mask.
According to the technique of Japanese Patent Application Laid-Open No. 2010-123937, the aforementioned processes are all performed by wet etching to form an end face into a tapered shape. As an etchant for this wet etching, a mixed liquid of phosphoric acid, acetic acid, and nitric acid, a mixed liquid of hydrogen peroxide, ammonia, and water, and ITO-07N (available from KANTO CHEMICAL CO., LTD.) as a commercially-available oxalic acid-based etchant are used properly.
If conventional amorphous silicon (Si) is used in a semiconductor active layer, a pattern is formed by dry etching. This facilitates formation of what is called a tapered shape of forming the cross section of a side wall corresponding to an end face of a semiconductor pattern into a gently sloping shape.
In contrast, if an oxide semiconductor film is used in a semiconductor active layer, a pattern is formed by wet etching. Wet etching is isotropic etching. Thus, a side wall of the patterned oxide semiconductor film becomes substantially vertical.
This may be a cause for a coverage failure of a film formed on the oxide semiconductor film. As an example, the step coverage characteristics of a TFT structure will be degraded at a source electrode and a drain electrode formed so as to cover a side wall of the pattern of the oxide semiconductor film and at respective side walls of a source wiring and a drain wiring, leading to a problem such as a break in a wiring film. This further causes the problem of degradation of the step coverage characteristics of a protective insulating film formed on the wiring film.
Using a film of metal such as aluminum (Al), titanium (Ti), chromium (Cr), copper (Cu), molybdenum (Mo), tantalum (Ta), tungsten (W), or an alloy of these metals for forming a source electrode and a drain electrode of a TFT causes a problem in that it becomes difficult to obtain favorable interfacial electrical characteristics stably at an electrical junction between the source electrode and the oxide semiconductor film and between the drain electrode and the oxide semiconductor film. Further, increase in interfacial electrical resistance causes a problem in that it hinders the oxide semiconductor film having high mobility from offering its performance significantly.
Japanese Patent Application Laid-Open No. 2010-123937 mentioned above discloses a technique responsive to the aforementioned problems. According to this technique, a side wall corresponding to an end face formed of a pattern of an oxide semiconductor film, that of an ohmic contact layer, that of a source electrode, and that of a drain electrode is tapered by wet etching.
However, Japanese Patent Application Laid-Open No. 2010-123937 does not disclose a specific method and a specific condition for the wet etching to become matters to be considered for formation of a tapered shape. Referring particularly to patterning of a multilayer film, unlike formation of a tapered shape using dry etching, formation of a tapered shape using wet etching results in a canopy structure (hereinafter called a “notch structure” in some cases), for example. This makes it difficult to form a side wall corresponding to an end face of a pattern into a shape realistically that enhances step coverage characteristics at respective side walls of a source electrode, a drain electrode, a source wiring, and a drain wiring.